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Iron-enrichment bottle experiments were monitored using flow cytometry to investigate the hypothesis that phytoplankton in the equatorial Pacific are iron-limited. Iron-enriched Synechococcus, ultraphytoplankton, nanophytoplankton, pennate diatoms, and coccolithophorids had higher fluorescence and/or forward light scatter per cell than control cells; for Prochlorococcus the trends were the same although the differences were not significant. This suggests that most phytoplankton cells were physiologically affected by the low iron concentrations in this region. However, only pennate diatoms showed significant increases in cell concentrations due to iron enrichment. The sum of chlorophyll fluorescences of individual cells measured by flow cytometry yielded patterns similar to those of extracted bulk chlorophyll, with increases of up to 10-fold in iron-enriched bottles but at most 3-fold in control bottles; pennate diatoms accounted for most of the increase in chlorophyll in iron-enriched bottles., Cited By (since 1996):39, CODEN: DSROE, ,